This project deals with the decoding of genetic information in translation. Specifically, the project focuses on the establishment of the roles of the genetic code by the aminocylation reactions that are catalyzed by aminoacyl tRNA synthetases. In these reactions, amino acids are matched with their cognate transfer RNAs which contain the anti- codon triplets of the code. The transfer RNAs are ancient molecules that are thought to have developed in an RNA world, while the synthetases were likely among the early proteins to emerge from an RNA world as the genetic code was established. Much effort is directed at understanding RNA-dependent amino acid discrimination in translational editing. In this reaction, the accuracy of the code is enhanced through an RNA- dependent refinement of the discrimination of closely similar amino acids. Here, the synthetase-tRNA complex functions in amino acid recognition as a ribonucleoprotein (RNP) that is perhaps reminiscent of an early development of synthetases as RNPs. A second goal is to understand how domains within a synthetase communicate, within the synthetase-tRNA complex. To a rough approximation, the two major domains of a tRNA interact separately with two domains in a tRNA synthetase. In particular, the primordial synthetase is thought to be represented by a catalytic domain that recognizes nucleotides determinants near the amino acid attachment site. This interaction is sufficient to catalyze aminoacylation of RNA oligonucleotide substrates known as microhelices that are based on just the accepted end of the tRNA. The relationship between nucleotide determinants in acceptor stems and the attached amino acid constitutes an operational RNA ode for amino acids that is distinct from the nucleotide triplets of the genetic code. For some synthetases the interaction of its second domain with the second anti-contain domain of the tRNA greatly enhances the rate of the aminoacylation by an unknown mechanism. A third goal is to see whether aminoacylated microhelix substrates can be used for peptide synthesis, in a ribosome- free system. Such a system could be representative of an early system for protein synthesis. Collectively, these investigations expand our understanding of the genetic code and the biochemical mechanisms that are its underpinnings. They also give clues into the possible connections between the RNA world an the theater of proteins. Because they are essential and show species-specific variations through evolution, the synthetases and tRNAs are ideal targets for therapeutic drugs directed at infectious pathogens. An expanded understanding of these systems could, therefore, have direct applications to human health.